The invention relates to systems and methods for selectively transmitting torque between rotatable members. More particularly, the present invention relates to a compliant clutch having a plurality of engagement members connected to a hub by flexible segments that permit extension of the engagement members to contact a rotatable receiving member, such as a drum.
In many mechanical devices, there exists a need to transmit torque in a variable fashion between two rotating members. Under certain circumstances, such as when a motor is idling or starting to operate, it may be desirable to have the motor disconnected from any load. However, the motor should be connected to the load during normal operation. For example, the engine of a car may be disconnected from the remainder of the vehicle""s drive train while idling at a stoplight, and may be reconnected to induce motion of the vehicle.
Several mechanisms exist for disconnecting and reconnecting a rotational load. For example, geared transmissions may disconnect a driving gear from a driven gear, thereby disconnecting a load from a motor. However, geared transmissions are somewhat complex, and typically require that the driving and driven gears be rotated at about the same rate of rotation before they can be reconnected. Additionally, a geared transmission is either fully connected or fully unconnected; there is no in-between state in which torque is transmitted, but relative slippage of the rotational members is still permitted.
Clutches have been developed to provide a more continuous torque transfer. Clutches utilize friction to gradually couple rotational members; since the coupling is not sudden, the rotational members need not be rotating at the same speed, or even in the same direction, for coupling to take place. The friction may operate to ultimately bring the rotational members to the same rotational speed, depending on how the clutch is designed.
Many different types of clutches exist. Some examples are rim types with internal or external expanding shoes, band types, disk or axial types, and cone types. Clutches may be engaged or disengaged manually by a user; for example, a manual transmission in a vehicle uses a clutch that can be selectively disengaged, typically by pressing a pedal. In the alternative, clutches may be engaged or disengaged automatically by some operating characteristic of the machine in which they are used. Centrifugal clutches, for example, may engage or disengage when a threshold rate of rotation of the clutch is achieved. Often, centrifugal clutches take the form of rim type clutches with internal expanding shoes that are spring loaded, so that the shoes contact the rim to transfer torque only when the centrifugal force is large enough to overcome the spring force on the shoes.
Known centrifugal clutches have a number of inherent disadvantages. First, they typically have many parts that must be separately produced and assembled. Each shoe must typically have at least one spring/pin joint combination, and several shoes will often be used; the total number of mechanical parts involved in the production of the centrifugal clutch can easily exceed twenty. Furthermore, known clutches are often quite thick; each pin joint must have a certain minimum length in order to operate. The thickness of the clutch, in combination with the multiplicity of parts required, makes the clutch somewhat heavy. The weight of the clutch contributes significantly to the mass moment of inertia of the entire rotational system, thereby decreasing the efficiency of the machine.
Furthermore, the torque capacity of the clutch depends on a number of factors, including the surface roughness values of the clutch and rim and the outward force with which the clutch presses against the rim. Although the shoes are typically contoured to match the rim, only parts of the shoe will contact the rim until the shoe wears somewhat; the shoes are worn to a smoother finish during use. Thus, the torque capacity will typically change somewhat during use and wear of the clutch. Even when a shoe is fully worn in, the pressure on the shoe is often concentrated at a comparatively small portion of the surface of the shoe. Increasing the number of shoes increases the number of parts, and is therefore a less desirable option for most mechanical clutches.
The manner in which the shoe moves to contact the rim also affects the torque capacity of the clutch. If a shoe moves such that the frictional force of the rim against the shoe tends to increase the pressure of the shoe against the rim, the shoe is termed an xe2x80x9caggressivexe2x80x9d shoe. Conversely, if the frictional force tends to actuate the shoe away from the rim, the shoe is a xe2x80x9cnon-aggressivexe2x80x9d shoe. Torque tends to tighten the engagement of aggressive shoes, thereby enhancing their torque capacity. However, aggressive shoes will generally have a far more sudden engagement than nonaggressive shoes; as a result, the aggressive centrifugal clutch does not gradually transfer torque to the load, but rather engages somewhat abruptly. Such abrupt engagement produces higher stresses and may damage mechanical components.
Furthermore, aggressive clutches that are designed or used improperly may induce a condition called xe2x80x9cself-locking.xe2x80x9d Self-locking occurs when the frictional force is sufficient, alone, to overcome the spring force and hold the aggressive shoes against the rim. When self-locking has occurred in a centrifugal clutch, the clutch may remain engaged, even when the clutch has slowed its rotation below the threshold rate of rotation. As long as the torque transferred by the clutch remains high enough to support self-locking, rotation of the clutch is no longer required for engagement.
A motor coupled to the clutch may thus be fully-loaded at a speed far lower than the minimum load-bearing speed of the motor. As a result, the motor or other mechanical components may suffer damage. Thus, the aggressiveness, and hence the torque capacity, of known clutches has also been limited by the need to design the clutch such that self-locking does not occur.
Although known compliant clutches have provided some improvements over known mechanical clutches. Compliant clutches utilize bending material in place of pin joints and springs to provide motion and restorative force. Thus, the part count, production expense, and weight can be decreased somewhat. Such compliant clutches have found application in the lawn and garden industry, in which many smaller machines such as string trimmers, hedge trimmers, edgers, and the like have a need for variable torque transmittal.
However, known compliant clutches typically have a low torque capacity, partly owing to the fact that the compliant clutches utilize a comparatively simple S-configuration with only two members that can bend outward to contact the rim. Thus, the degree of friction that can be generated by known compliant clutches is quite limited. Additionally, torque is often transmitted through the thinner, compliant members of the clutch, so that the strength of the material used to form the compliant members limits the torque the clutch can effectively handle. In many applications, multiple compliant clutches must be used to generate the necessary torque.
Consequently, there is a need in the art for a clutch that is easily manufactured from a small number of parts, and with a minimum of assembly. Furthermore, there is a need in the art for a clutch that can fit within a compact space, and yet provide a high torque capacity. Such a clutch should preferably provide a torque capacity that is high even before significant wear of the clutch has occurred, and that changes comparatively little when wear occurs. There is a further need for a centrifugal clutch in which torsional stress is not concentrated in thin, compliant members of the clutch. The high torque capacity should preferably be provided while maintaining a comparatively smooth engagement, and avoiding any danger of self-locking.
The present invention is directed to compliant clutches with enhanced load-bearing, wear, and manufacturability. The clutches of the present invention may be designed to operate as part of a torque transfer system, in which the clutch resides within a receiving member, which may be configured as a drum with a cylindrical interior surface. The clutch may be attached to a first rotational member, and the drum may then be attached to a second rotational member.
Although the clutch or drum may be connected to other rotating components by gears, belts, magnetic couplings, or the like, the rotational members may simply take the form of a first shaft and a second shaft. The first shaft may be connected to a torque source, such as a rotary motor, and the second shaft may then be connected to a load, such as a generator, flywheels, vehicle wheels, helicopter blades, or the like.
In selected embodiments, the clutch may have a plurality of engagement members connected by flexible segments integrally formed with the engagement members. Each engagement member may have an outer edge shaped to engage the interior surface. The engagement members are preferably situated around a hub that can be affixed to the first shaft. The hub, flexible segments, and engagement members may then be configured in a wide variety of ways.
According to one presently preferred embodiment, the engagement members and flexible segments are detached from the hub. The engagement members are arrayed in ring-like fashion around the hub, with each engagement member connected to its two contiguous neighbors by flexible segments. The engagement members and flexible segments may then be referred to as an expandable engagement portion of the clutch. The flexible segments are alternatingly disposed near the hub, and near the outer edges. As a result, when the flexible segments are rotated so that the inner flexible segments are moved outward, the entire expandable engagement portion may expand so that the outer edges contact the interior surface of the drum.
Due to the arrangement of the flexible segments, expansion of the expandable engagement portion causes each engagement member to pivot in a direction opposite that of its nearest neighbors. However, the frictional force that acts against the engagement members when the outer edges contact the interior surface acts to rotate all of the engagement members in the same direction. Thus, the frictional force tends to push half of the engagement members out of engagement with the interior surface, half of the engagement members into tighter engagement with the interior surface.
As a result, half of the engagement members are aggressive, while half are non-aggressive. All of the engagement members are coupled together so that they cannot independently move toward or away from the interior surface. Thus, a comparatively high torque capacity is provided by the aggressive engagement members, while the non-aggressive engagement members ensure that engagement is relatively smooth. Self-locking is unlikely because the non-aggressive engagement members counteract the frictional forces that would tend to cause self-locking of the aggressive engagement members.
Preferably, the hub has a plurality of arms extending outward into interior slots of the expandable engagement portion. Thus, torque may be transferred directly from the hub into the engagement members, and from the engagement members to the drum. Although some torque may be transferred through the flexible segments, torsional stresses in the clutch are not all concentrated in the comparatively thin, compliant segments, but are rather transmitted through other pathways. Thus, the torque capacity is not materially limited by the strength of the flexible segments.
For purposes of analysis, each aggressive engagement member may be paired with a non-aggressive engagement member to form an engagement pair. The hub may have arms separating each engagement pair; the arms may act to transfer torque between the hub and the expandable engagement portion. The arms may thus fit within interior slots formed in the expandable engagement portion, between each engagement pair.
Since all of the engagement pairs are symmetrically arrayed about the hub, analysis of a single engagement pair can be carried out and applied to the entire expandable engagement portion. Such analysis may be relatively easily performed through the aid of a pseudo-rigid body model (PRBM). Large deflection of members is difficult to determine analytically. However, in the PRBM, the flexible segments are approximated as pin joints with attached torsional springs. Shorter flexible segments have the pin joint positioned in the middle of the flexible segment, while longer flexible segments may have a pin joint positioned at a predetermined distance from a stationary end of the flexible segment.
Through the use of such approximations, the operation of each engagement pair may be analyzed using traditional kinematic analysis techniques. Thus, the clutch may be modified to obtain desired operational characteristics. For example, it may be desirable to obtain a clutch with a certain threshold rotational rate, or a series of varying torque capacities over a range of rotational rates. The clutch may need to have a certain diameter, thickness, weight, or wear resistance. Analysis of the PRBM is helpful in determining how thick and long the flexible segments should be, what the mass of the engagement members should be, where the centers of gravity of the engagement members should be located, what materials the clutch should be made of, and other critical parameters. These parameters can then be used to obtain an optimally-designed compliant centrifugal clutch.
Such a clutch may also be relatively easily manufactured. According to a preferred embodiment, the engagement members, the flexible segments, and the hub all lie and move within the same plane. Thus, the clutch can be manufactured by processing a workpiece of the proper material with a simple, planar, manufacturing operation. The centrifugal clutch may, for example, be milled, stamped, molded, extruded, or the like. The hub may be made from a cutout of the expandable engagement portion. Preferably, the clutch is made symmetrical, so that the clutch can be used in the same way in either rotational direction.
According to one alternative embodiment, the engagement members, flexible segments, and the hub may all be formed unitarily. The centrifugal clutch may once again have engagement members connected such that half of the engagement members move in an aggressive manner, and half move in a non-aggressive manner. Rather than being connected directly to each other by flexible segments, each engagement pair may connected to the hub by the flexible segments. Furthermore, each engagement pair may be connected to the hub by two separate flexible segments positioned at either end of the engagement pair. Thus, each engagement pair is redundantly connected to the hub to form a closed loop. The engagement pairs, with their associated flexible segments, may collectively be referred to as an extensible engagement portion.
The hub may have arms extending outward, from which the flexible segments extend to reach the engagement members. Between the arms, each engagement pair may have a flexible segment connecting the aggressive engagement member to the non-aggressive engagement member, in a position close to the hub. Thus, the motion of the engagement members may be similar to that of the engagement members of the first embodiment.
The clutch may operate in somewhat similar fashion to that of the first embodiment. The aggressive and non-aggressive engagement members may be induced by friction to rotate in opposite directions about their own centers of gravity. Thus, a comparatively high torque capacity may still be obtained without sacrificing engagement smoothness. Advantageously, the clutch with an integral hub requires no mechanism to keep the hub and the extensible engagement portion coplanar.
The clutch according to such an embodiment may be analyzed in much the same fashion as described in connection with the first embodiment. A pseudo-rigid body model may be created based on the shape of the clutch, and kinematic analysis may be carried out according to traditional methods to determine the necessary parameters for the clutch. The clutch may also be manufactured using a single planar operation, as described in connection with the previous embodiment.
According to yet another alternative embodiment, a centrifugal clutch according to the invention may have only non-aggressive engagement members, and may be designed to rotate in a single direction. For example, multiple engagement members may be connected together by one or more flexible segment to form a first arm, which may then be connected to the hub by another flexible segment. The hub may have an arm extending away from the hub, and the first arm may be attached to the arm of the hub in trailing fashion (such that the arm of the hub leads the first arm in its rotation). Preferably, the centrifugal clutch also has a second arm symmetrical with the first arm, so that the clutch is rotationally balanced on the first rotatable member. According to selected embodiments, each of the first and second arms has at least two engagement members connected by flexible segments. Preferably, each arm has from two to five engagement members connected by flexible segments.
In operation, the clutch in this embodiment may provide an exceptionally smooth engagement. The engagement members disposed at the end of the first and second arms may be the first to contact the interior surface. As the clutch rotates faster, the other engagement members may come into contact with the interior surface one-by-one, so that the amount of friction between the clutch and the drum increases gradually with the angular velocity of the clutch. The engagement of all of the engagement members is non-aggressive because the frictional force acting on each engagement member tends to rotate the engagement members inward, away from the interior surface. The comparatively large number of engagement members tends to increase the frictional force exerted by the clutch against interior surface, thereby providing an increased torque capacity over known clutch designs having only two rim-engaging surfaces.
Preferably, the first and second arms, including the engagement members and the flexible segments, are integrally formed with the hub. Once again, planar manufacturing methods can be used to create the clutch. Additionally, a pseudo-rigid body model may be utilized to analyze the operation of a design for the clutch, and the adjust design parameters of the clutch accordingly.
Through the novel clutch designs presented herein, the torque capacity of centrifugal clutches may be enhanced without sacrificing starting smoothness or creating a significant risk of self-locking behavior. Clutches may be easily designed to suit a wide variety of applications through the application of pseudo-rigid body modeling techniques. Additionally, the clutches of the present invention fit within a small space, and may be readily manufactured using rapid and inexpensive processes.